<unistd.h>: explain why getpagesize() is better than getauxval() as well as sysconf().

I thought we'd already done this, but apparently not.

Bug: http://b/315511637
Change-Id: I0581438a6f551be9750b6679a9f3ad06c83f1e9b

libc/include/unistd.h

@@ -353,7 +353,7 @@ int acct(const char* _Nullable __path);
 /**
  * [getpagesize(2)](https://man7.org/linux/man-pages/man2/getpagesize.2.html)
  * returns the system's page size. This is slightly faster than going via
- * sysconf().
+ * sysconf(), and avoids the linear search in getauxval().
  *
  * Returns the system's page size in bytes.
  */